Radioprotective agents, also known as radioprotectors, are defined as agents which protect cells or organisms from deleterious cellular effects of exposure to ionizing radiation. These deleterious cellular effects include damage to cellular DNA, such as DNA strand break, disruption in cellular function, cell death, tumor induction and the like. The mechanism of this protective effect may at least partially be due to radical scavenging properties of the radioprotective agents.
The potential utility of these agents in protecting against exposure to environmental radiation, as well as in cancer radiation therapy, has long been recognized. These agents, administered prior to or during exposure, would eliminate or reduce the severity of deleterious cellular effects caused by exposure to environmental ionizing radiation such as resulting from a nuclear explosion, a spill of radioactive material, close proximity to radioactive material and the like.
In addition, these agents are believed to provide a selective protection of normal cells, and not of cancer cells, during cancer radiation therapy. For example, these agents, administered to the cancer patient prior to or during radiation therapy, will be absorbed by normal, non-cancer cells to provide a protective effect. However, the radioprotective agents will not be absorbed to the same extent by tumor cells due to the poor vascularity associated with the tumor. Therefore, the radioprotective agents would provide a selective protective effect on the normal cells as compared to tumor cells and would eliminate or reduce the severity of deleterious cellular effects of radiation therapy on normal cells. Furthermore, some radioprotective agents may act as prodrugs and require activation by cellular enzymatic processes which are not fully operative in the cancer cell. These agents, even if absorbed in a similar concentration in normal and cancer cells, will only be activated in cells with normal enzymatic processes and not in cancer cells. These prodrug radioprotective agents would be activated to provide a selective protective effect only in normal cells and would thus eliminate or reduce the severity of deleterious cellular effects of radiation therapy on normal cells.
Furthermore, certain radioprotective agents provide a selective protection against deleterious cellular effects in normal cells caused by certain DNA-reactive agents such as cisplatin, cyclophosphamide, diethylnitrosoamine, benzo(a)pyrene, carboplatin, doxorubicin, mitomycin-C and the like. Many of these DNA-reactive agents are chemotherapeutic agents useful in cancer therapy. Radioprotective agents are useful in eliminating or reducing the severity of deleterious effects in normal cells caused by exposure to these DNA-reactive agents, such as during cancer therapy with DNA-reactive chemotherapeutic agents.
In addition, certain radioprotective agents provide a selective protection against therapy-induced secondary tumor induction [See Grdina et al., Pharmac. Ther. 39, 21 (1988)]. Radiation and chemotherapy provide effective treatments for a variety of neoplastic disease states. Unfortunately, these treatments themselves are oftentimes mutagenic and/or carcinogenic and result in therapy-induced secondary tumor induction. For example, patients treated for Hodgkin's disease appear to exhibit a relatively high risk for therapy-induced acute myelogenous leukemia and non-Hodgkin's lymphoma. Radioprotective agents provide selective protection against deleterious cellular effects, such as tumor induction, caused by radiation therapy or chemotherapy with a DNA-reactive chemotherapeutic agent. Radioprotective agents are thus useful in eliminating or reducing the risk of secondary tumor induction brought about by radiotherapy or chemotherapy.
Radioprotective agents thus are useful in eliminating or reducing the severity of deleterious cellular effects in normal cells caused by environmental exposure to ionizing radiation, cancer radiation therapy and treatment with DNA-reactive chemotherapeutic agents. See generally, Weiss and Simic, Pharmac. Ther. 39, 1 (1988).
The prototypical radioprotective agent, developed by the Antiradiation Drug Development Program at the Walter Reed Army Institute of Research, is WR-2721, or S-2(3-aminopropylamino)ethylphosphorothioic acid, which has the structure EQU H.sub.2 N--(CH.sub.2).sub.3 --NH--(CH.sub.2)2 --S--PO.sub.3 H.sub.2WR- 2721.
Other known radioprotective agents are WR-1065, thought to be a metabolite of WR-2721, which has the structure EQU H.sub.2 N--(CH.sub.2).sub.3 --NH--(CH.sub.2).sub.2 --SH WR-1065,
and WR-151,327, which has the structure EQU CH.sub.3 NH--(CH.sub.2).sub.3 --NH--(CH.sub.2).sub.3 --SPO.sub.3 H.sub.2WR- 151,327.